Thin film transistor is an important component in a liquid crystal display device. Currently, in the TFT techniques, active layers are most made of amorphous silicon (α-Si) or polysilicon (p-Si). With development of display technology, demands on display panel of a greater size and high resolution are continuously increasing. α-Si is limitative to be applied in display panels with great size, high resolution and fast response speed, due to its low mobility value (0.5˜0.8 cm2/v*s). Even though polysilicon has a high mobility value (>10 cm2/v*s), processes for manufacturing a display panel with polysilicon is very complicated and processes for manufacturing a display panel with great size has not been well developed, application of polysilicon is also limited in display panels with great size and high resolution. Comparatively, oxide semiconductor has a promising perspective in manufacturing display panels with great size and high resolution due to its high mobility value (>10 cm2/v*s) and its applicability in manufacturing display panels with great size.
In conventional processes for manufacturing oxide semiconductor TFT array substrate, because of limitation on processes and selected materials, an oxide semiconductor layer is prone to be exposed to ambient environment and its electrical conductivity will be degraded. Contact resistance between the oxide semiconductor layer and source/drain electrodes is relatively high, which requires high driving voltages for the display device and consumes more energy, and robustness of the formed TFT is degraded simultaneously.